The invention relates to a method of effecting a motion filtering for detecting point targets moving slowly within a time sequence of readjusted binary images, these images corresponding to a history at the instant n-1, denoted Hist n-1, including target trajectories and alarms.
The invention is applicable for example in the monitoring of road traffic.
There is already known from the state of the art a so-called Kalman filtering method based on the prediction of trajectories. However, in its current usage the Kalman method is very complicated since it generates a large number of paths in order to try to connect each trajectory with each one of the new alarms which appear. It is therefore very expensive in terms of computation time and in memory. On the other hand, it can be used for rapidly moving targets, that is to say assuming a movement greater than 10 pixels between two successive images.
One of the objects of the invention is to provide a method applicable to the detection of slow targets, and which gives results which are all the better the slower the targets.
Another object of the invention is to provide such a method which is inexpensive in terms of computation time and memory.
Another object of the invention is to provide a method which is easy and inexpensive to implement.
Now, the detection of slow targets presents a certain number of problems which the invention proposes to solve in order to achieve its objects.
It will be noted that the detection of motion in a sequence of images implies firstly determining a difference between two consecutive images, in such a way as to show up the moving portions and the stationary portions. However this difference, termed first order difference, does not provide sufficient elements to produce the detection of point targets in a sequence of images, when these targets have a slow motion. difference image there are contributions due to noise, which may give the impression of moving objects or of targets.
Secondly, the images must be readjusted. This is required because, in general, the camera which acquires the images to be treated is not sufficiently stable. It is itself mobile, either if it is mounted on a moving vehicle, or it is mounted on a structure which may oscillate for example in the wind. The images acquired are therefore readjusted relative to one another in such a way that the background, for example, appears stationary in the sequence of images. The readjustment is effected by a method known per se, and which does not, therefore form part of the invention. However, it appears that the readjustment is never perfect. It generally leads to readjustment errors of the order of a pixel, which means that between two consecutive images, there appear systematic errors of the order of a pixel where corresponding portions of the images coincide. As a result of this, in the difference image, the contribution is found from these readjustment errors which can likewise be mistaken for moving objects. On the other hand, in the method of readjustment, there may moreover appear occasional sizable errors extending over a not insignificant proportion of successive images in the sequence of images. The contribution from these occasional errors, which are due to the method of readjustment, are also a source of errors in the detection of slow moving targets.
Thirdly, among the objects which may appear in the images, there are a certain number of targets which it is not wished to detect. These are for example, trees or clouds in motion, or portions of the background subject to variations in lighting. On the contrary, one of the aims of the invention is the detection of moving bodies having relatively continuous and relatively uniform trajectories.